Preheat oven to 350 degrees F. Line two baking sheets with parchment paper.
In a large mixing bowl, whisk together powdered sugar, cocoa powder, cornstarch and salt until well blended. Add in egg whites, egg and vanilla extract and mix until well combined. Fold in chocolate chips.
Drop dough by the heaping tablespoonfuls onto prepared baking sheets, spacing cookies 2-inches apart.
Remove from oven and sprinkle tops lightly with Maldon sea salt if desired. Allow to cool on baking sheet 5 - 10 minutes, then cool on a wire rack.